Well, I just copy any map (depending from size of it), then just delete all objects, use flat brush to level all heights, paint everything to same texture and get a clear flat map. Some things may work badly, tho. I used aleppo_border map as sample and got many enemy units turn invisible and ♥♥♥♥. But palmyra_airport map worked well and I had no problems editing it.

You can use the landscape from latakia_border template, it is completely flat. You just need to
repaint it to the desert.

Thank you guys.